Summer monsoon variations over the last 1.2 Ma from the weathering of loess-soil sequences in China

The loess-soil sequence in northern China contains a near continuous record of Quaternary paleoclimate. Magnetic susceptibility and grain size have so far been the only proxies available to address the long-term changes of the East-Asian paleomonsoon extending back to more than one million years. In this study, the ratio of CBD (citrate-bicarbonate-dithionite)extractable free Fe2O3 (FeD), a measure of iron liberated by chemical weathering, versus the total Fe2O3 available (FeT) was measured on samples at 10 cm intervals taken from two loess sections deposited over the last 1.2 Ma. Variations of this index are highly consistent with other pedological indicators, but in addition provide a quantitative measurement of the degree of pedogenesis in the Loess Plateau. Since chemical weathering in the region mainly depends upon summer precipitation and temperature, weathering intensity primarily reflects changes in the East-Asian summer monsoon. The new proxy has been used to document a series of summer monsoon changes of global significance, which are not necessarily recorded by magnetic susceptibility.